Process of isolating rare earth elements

ABSTRACT

A process of isolating a REE or a group of REE from a solution or dispersion containing said REE or said group of REEs comprising the following steps: (i) preparing a mixture comprising said solution or dispersion and biomass comprising at least one organism selected from any one of the following organism classes: eubacteria, archaea, algae, and fungi, whereby the at least one organism is capable of adsorbing or accumulating said REE or said group of REEs; (ii) incubating said mixture of step (i) for allowing the adsorption or accumulation of said REE or said group of REEs by said biomass; (iii) separating the biomass having adsorbed or accumulated REE(s) from the mixture of step (ii); and (iv) isolating said REE or said group of REEs from said biomass separated in step (iii).

FIELD OF THE INVENTION

The invention relates to a process of isolating a rare earth element(REE) or a group of REEs from an aqueous solution or dispersioncontaining the REE or the group of REEs. Rare earth elements areisolated and concentrated from an aqueous solution by adsorption oraccumulation by microorganisms and are subsequently released therefromby separation procedures. The invention also provides a method oftesting a sample microorganism for its ability to bind a rare earthelement or of screening a plurality of microorganisms for the ability ofmembers of said plurality to bind a rare earth element.

BACKGROUND OF THE INVENTION

According to a recent classification the rare earth elements include the17 elements scandium, lanthanium, cer, praseodym, neodym, promethium,samarium, europium (light rare earth elements, LREE) and yttrium,gadolinium, terbium, dysprosium, holmium, erbium thulium, ytterbium,lutetium (heavy rare earth elements, HREE). The demand for rare earthelements is growing steadily due to their importance, particularly inthe field of high-tech electronics and displays. Further, a relevantshare of the increasing demand is caused by so-called “greentechnologies” which aim at the reduction of energy consumption,development of renewable energy carriers and air pollution control. Forexample, rare earths are used in wind turbines, (hybrid) electricvehicles, automotive catalysts and energy-efficient lighting systems.Due to the increasing demand there is an urgent need for more efficientand sustainable mining processes even from low grade ores as well asefficient methods for recycling of rare earth metals, i.e. recyclingthem from waste material such electronic or metal scrap. To date, therehas been no large scale recycling of REE from magnets, batteries,lighting and catalysts although the amounts of waste are substantial.The advantages of recycling REEs are amongst others the lack ofradioactive impurities and economic independence from supply fromprimary sources. One reason for inadequate exploitation of thesevaluable resources, sometimes termed “urban mining”, is that recyclingprocesses for REEs are quite complex and energy-consuming, comprisingphysical and chemical treatment and generally the available know-how isstill quite low.

Conventional Beneficiation Processes

In most of the processes for REE beneficiation, the ore is mined in thedeposit, broken down or milled and REE minerals are concentrated usingphysical properties such as density, magnetism and surface activitieslike electrostatic charge or flotation efficiency. The thus concentratedore is then leached and the resulting REE-bearing solution is purifiedfrom undesired elements such as Fe, Ca, thorium and uranium. The processis described in e.g. Gupta, C. K. and Krishnamurthy, N. (2005):Extractive Metallurgy of Rare Earths, CRC Press (Florida, USA), andCastor, S. B. and Hedrick, J. B. (2006): Rare Earth Elements, In: Kogel,J. E., Trivedi, N. C., and Krukowski, S. T. (eds.): Industrial mineralsand rocks: Commodities, markets, and uses. 7th edition, SME, page769-792, as well as references therein. However, this leaching or‘cracking’ stage depends on the type of minerals and othercharacteristics of the deposit. For instance at Mountain Pass, themineral concentrate was calcined to drive off CO₂ and fluorine andleached with HCl to dissolve most of the trivalent REEs. Whereas theREE-bearing liquid was used for the separation of individual REEs, theresidue (predominantly CeO₂) was sold. In contrast to that, the BayanObo REE mineral concentrate is baked with sulphuric acid at 300° C. too600° C. and leached with water, taking REEs into solution andprecipitating other elements as waste REEs are then precipitated asdouble sulphates and converted to hydroxides (representing a chemicalmixed rare earth concentrate), which are leached with HCl for theseparation of individual REE. The varying composition and distributionof the individual REEs in the pregnant, REE-bearing solution or in theprecipitated chemical mixed rare earth concentrate depends on themineral deposit from which the ore originates. Following the leachingstage subsequent processing is required to separate individual REEs fromeach other. Separating individual REEs is a very difficult process dueto their similar chemical properties. As a consequence the high value ofREEs depends on their effective separation into high purity compounds.Cerium and europium can be separated by selective oxidation or reductionwhilst other REEs can be separated in small amounts using fractionalcrystallisation or fractional precipitation. However, commercialseparation generally is done using solvent extraction and, less common,ion exchange methods. Solvent extraction (SX), or liquid-liquidextraction, is a method used to separate compounds on the basis of theirrelative solubility's in two immiscible liquids, commonly theREE-containing aqueous solution and an organic solvent. On an industrialscale the solvent extraction is carried out in a group of mixersettlers, which allows repetitive fractionation during a continuouslyflowing process. Initially the process is relatively ineffective. Whenthe process is repeated many times each REE can be separated from theothers. However, the solvent extraction method is most appropriate forseparating the LREEs, with the HREEs being more difficult to extractusing this method. This is especially true if the used ore consistspredominantly of the LREEs. Ion exchange is a process in which ions areexchanged between a solution and an insoluble (usually resinous) solid.The REEs from the solution displace the cations on the resin surface,whereas the aqueous waste containing the exchanged cations. IndividualREEs are then separated using a complexing agent which has differentaffinities for the various REEs. The Ion exchange method produces highlypure REE in small quantities. However, it is a time consuming and thusexpensive process. Consequently, only a small amount of HREEs arepurified commercially on a small scale using ion exchange.

Impact of Conventional Rare Earth Leaching on the Environment and onHealth

The main environmental risks of conventional processes are due totailings containing small-size particles, waste water and flotationchemicals. The tailings typically remain in the impoundment areas wherethey are continuously exposed to water e.g. from rain. Toxic substancesare washed out, producing steady emissions to ground water. Thecomposition of the polluting water is site-specific depending on thehost minerals and the chemicals used for leaching and flotation. Thetailings may contain radioactive substances, arsenic, fluorides,sulfides, acids and heavy metals. The refining of the rare earthconcentrate is an energy-intensive and water consuming process andcauses serious air emissions e.g. of SO₂, HCl, dust. Additionally, thesolvent extraction method causes waste water, which is often extensivelypolluted by organic solvents like kerosene. Additionally, radioactivewaste can arise, as the majority of rare earth deposits also containthorium and/or uranium, thus radionuclides may pollute water and air.CO₂-emissions are also significant.

Bioleaching

The possibility to use predominantly acidophilic, autotrophiciron-oxidising sulfur-oxidizing prokaryotes to recover precious and basemetals from mineral ores and concentrates is known, e.g. from Rawlings,D E and Johnson, D B (The microbiology of biomining: development andoptimization of mineral-oxidizing microbial consortia, Microbiology2007, 153: 315-24). The development of this technology was inspired bythe observation that certain bacteria, especially Thiobacilli, are ableto solubilise heavy metal minerals by oxidizing Fe(II) to Fe(III) aswell as sulfidic compounds to sulfate. This process is the major causeof natural weathering of sulfidic minerals.

By creating conditions that favour the growth of ore-decayingmicroorganisms, leaching of heavy metals from sulfidic minerals underaerobic conditions can be increased more than 100-fold compared toweathering without bacteria. However, degradation of minerals such aspyrite enclosing precious metal atoms or clusters can lead to therelease of the trapped high-value compounds. While cheap in situ ordump/heap set-ups are generally used to bioleach base metals fromlow-grade rocks and minerals, more expensive (and more controlled)stirred-tank reactors are typically employed in the pre-treatment ofmineral ores for the recovery of metals. After the initial bacterialdisintegration step, ores are subjected to a conventional chemicalleaching process, hazarding the environmental and health problemsmentioned above.

Bioadsorption

A number of living microorganisms, but also nonviable, inactivated cellshave the ability to bind metal ions. In the first case, metal bindingcan occur via adsorption to the cell surface or via active intracellularaccumulation of metal ions. In the latter case of nonviable, inactivatedcells—that is often referred to as biosorption—metal ion binding isbelieved to occur exclusively via surface adsorption. The biosorptioncapacity as a general characteristic of biomass results from thepresence of chelating groups (e.g. carboxyl-, amide-, hydroxyl-,phosphate-, and thiol-groups) contributed by carbohydrates, lipids andproteins that are displayed on the cell surface. It has been describedthat amounts of metals of up to 50% of the cell dry weight can beaccumulated by biomass (Vieira and Volesky, 2000). United States Patent1991/5055402 describes a process for removing metal ions from aqueoussolution, using a matrix prepared from metal-binding microorganisms thathave been heat-inactivated at temperatures of 300-500° C. However,specific binding mechanisms by organic surface structures are obviatedby this procedure. EP 0673350 B1 describes the accumulation of metals,including some rare earth elements such as lanthanium and yttrium, byreacting phosphate ions generated by a microorganism and metals topolyphosphates. Accumulation of the metal-poly phosphates by themicroorganism of the genus Acmetobacter makes the metals accessible toprecipitation and depletion thus enabling purification of metal-pollutedwater. WO 1891/003424 describes a biomining procedure for leaching ofgallium and germanium from ores using an admixture of bacteria, culturemedium and crushed ore. However, no process has been described to datethat could be used to recover REEs in significant amounts from ores orwaste materials.

It is therefore an object of the present invention to provide a processof recovering, enriching or isolating REEs from source material, such asa mineral ore or a waste material containing REEs. It is another objectof the invention to provide a process for modulating the composition ofREEs in a solution, or isolating a particular REE, such as scandium orlutetium, from a solution. It is another object to provide organisms forthese processes. It is a further object to provide a screening methodfor such organism.

SUMMARY OF THE INVENTION

These objects are accomplished by:

-   -   (1) A process of isolating a rare earth element (REE) or a group        of REEs from a solution or dispersion, comprising the following        steps:        -   (i) preparing a mixture comprising a solution or dispersion            containing said REE or said group of REEs and biomass that            comprises at least one organism selected from any one of the            following organism classes: eubacteria, archaea, algae, and            fungi, whereby the at least one organism is capable of            adsorbing or accumulating said REE or said group of REEs;        -   (ii) incubating said mixture of step (i) for allowing the            adsorption or accumulation of said REE or said group of REEs            by said biomass;        -   (iii) separating the biomass having adsorbed or accumulated            REE(s) from the mixture of step (ii); and        -   (iv) isolating said REE or said group of REEs from said            biomass separated in step (iii).    -   (2) The process according to (1), wherein said solution or        dispersion is obtained by treating a source material of said REE        or said group of REEs, such as a mineral ore, a mineral mining        waste material, or an electronic or metal scrap, by an        extraction or solubilising agent, such as an aqueous acid.    -   (3) The process according (2), wherein obtaining said solution        or dispersion from said source material comprises pre-treating        said source material with auto- or heterotrophic bacteria for        bioleaching said REE or group of REEs from said source material        before or concurrently to step (i).    -   (4) A process of isolating or enriching a rare earth element        (REE) or a group of REEs from a particulate material, comprising        the following steps:        -   (i′) preparing a mature comprising a solution or dispersion            prepared from said particulate material and biomass            comprising at least one organism selected from any one of            the following organism classes: eubacteria, archaea, algae,            and fungi, whereby the at least one organism is capable of            adsorbing or accumulating said REE or said group of REEs;        -   (ii) incubating said mixture of step (i) for allowing            adsorption or accumulation of said REE or said group of REEs            by said biomass;        -   (iii) separating the biomass having adsorbed or accumulated            REE(s) from the mixture of step (ii); and        -   (iv) isolating said REE or said group of REEs from said            biomass separated in step (iii).    -   (5) The process according to (4), wherein said particulate        material containing said REE or said group of REEs is a        particulate mineral ore, a particulate mineral mining waste        material or a particulate obtained from electronic scrap or        scrap metal.    -   (6) The process according to (4) or (5), wherein said        particulate material contains said REE(s) in the form of        chemical compounds of the REE(s), such as carbonates sulfates,        oxides, phosphates or silicates.    -   (7) The process according to any one of (4) to (6), wherein said        particulate material has an average particle size of at most 5        mm, preferably at most 1 mm, more preferably of at most 400 μm,        and most preferably of at most 100 μm.    -   (8) The process according to any one of (4) to (7), wherein said        particulate material is pre-treated with sulfide-oxidising        bacteria for bioleaching said REE or group of REEs from said        particulate material before or concurrently to step (i).    -   (9) The process according to (3) or (8), wherein said        sulfide-oxidising bacteria are genetically-modified to express        an S-layer on the surface of said bacteria.    -   (10) The process according to any one of (4) to (9), wherein        said solution or dispersion is obtained by treating said        particulate material by an extraction or solubilising agent such        as an aqueous acid, optionally followed by adjusting the pH to a        suitable pH for adsorption or accumulation of said REE or said        group of REEs by said biomass.    -   (11) The process according to any one of items (4) to (10),        wherein preparing said solution or dispersion from said        particulate material composes pre-treating said particulate        material with autotrophic or heterotrophic bacteria for        bioleaching said REE or group of REEs from said particulate        material before or concurrently to step (i).    -   (12) The process according to any one of (1) to (11), wherein        said solution or dispersion is aqueous.    -   (13) The process according to (12), wherein said aqueous        solution or dispersion has a pH of from 0 to 5, preferably of        from 0.5 to 3, more preferably of from 1.0 to 2.0; and/or said        mixture of step (ii) has a pH of from 0 to 5, preferably of from        0.5 to 3, more preferably of from 1.0 to 2.0.    -   (14) The process according to any one of (1) to (13), wherein        step (ii) comprises agitating said mixture for bringing said        biomass in close contact with REE(s) present in said mixture.    -   (15) The process according to any one of (1) to (14), wherein        said REE is selected from the group consisting of scandium,        lanthanium, cer, praseodym, neodym, promethium, samarium,        europium, yttrium, gadolinium, terbium, dysprosium, holmium,        erbium, thulium, ytterbium, and lutetium; or said group of REEs        comprises at least two REEs selected from the aforementioned        list.    -   (16) The process according to any one of (1) to (14), wherein        said REE is selected from the group consisting of lanthanium,        cer, praseodym, neodym, promethium, samarium, and europium; or        said group of REEs comprises at least two REEs selected from the        aforementioned list.    -   (17) The process according to any one of items (1) to (14),        wherein said REE is selected from the group consisting of        terbium, dysprosium, holmium, erbium, thulium, ytterbium,        lutetium, scandium, yttrium; or said group of REEs comprises at        least two REEs selected from the aforementioned list.    -   (18) The process according to any one of (1) to (14), wherein        said REE is selected from the group consisting of scandium,        yttrium, gadolinium, terbium, dysprosium, holmium, erbium,        thulium, ytterbium, lutetium; or said group of REEs comprises at        least two REEs selected from the aforementioned list.    -   (19) The process according to any one of (1) to (14), wherein        said REE is scandium.    -   (20) The process according to any one of (1) to (19), wherein        the biomass is separated in step (iii) by one of the methods        selected from centrifugation, filtration, flocculation and        flotation.    -   (21) The process according to any one of (1) to (20), wherein        step (iii) involves blowing of air into said mixture for        accumulating biomass having bound REE at the surface of said        mixture.    -   (22) The process according to any one of (1) to (21), wherein        the incubation time of step (ii) is between 0.5 hours and 96        hours, preferably between 0.5 hours and 48 hours, and most        preferably between 1 hour and 24 hours.    -   (23) The process according to any one of (1) to (22), wherein        said organism is from genus Cupriavidus, such as Cupriavidus        metallidurans, or said organism is Citrobacter sp.    -   (24) Use of biomass selected from the following organism        classes: Eubacteria, Archaea, Algae, and Fungi for isolating a        REE or group of REEs from a particulate material or a solution        or dispersion containing said REE or said group of REEs. In one        embodiment, the organism is from genus Cupriavidus, such as        Cupriavidus metallidurans, or said organism is Citrobacter sp.    -   (25) A method of testing a sample microorganism for its ability        to bind a rare earth element or of screening a plurality of        microorganisms for the ability of members of said plurality to        bind a rare earth element, comprising the following steps:        -   (a) contacting a microorganism with a solution or dispersion            containing a rare earth element (REE) or multiple REEs;        -   (b) incubating the mixture of step (a) for a predetermined            period of time;        -   (c) separating microorganisms from the mixture obtained in            step (b);        -   (d) analysing the separated microorganism for bound REE.    -   (26) The method of (25), wherein said organism is selected from        the following organism classes: Eubacteria, Archaea, Algae, and        Fungi.    -   (27) The method of (25) or (26), wherein said solution or        dispersion used in step (a) is an acidic aqueous solution or        dispersion having a pH of from 0 to 5, preferably of from 0.5 to        3, more preferably of from 1.0 to 20; and/or wherein the mixture        of step (b) has a pH of from 0 to 5, preferably of from 0.5 to        3, more preferably of from 1.0 to 2.0.    -   (28) The method according to any one of (25) to (27), wherein        said predetermined period of time is between 0.5 hours and 96        hours, preferably between 0.5 hours and 48 hours, and most        preferably between 1 hour and 24 hours.

The inventors have found that REEs can be isolated or enriched from asolution or dispersion, preferably an aqueous solution or dispersion,using biomass comprising organisms that can bind REEs. The inventionprovides a process for isolating REEs in a simple and cost-effectiveway. The biomass binds the REE or a group of REEs by cell components ofthe organisms. After separation of the biomass from unbound material,the REEs can be isolated from the biomass. The invention allowsisolating REEs from sources that contain only low amounts of REEs,reducing the number of steps needed for REE separation compared to aconventional multistep process (refining or raffination). Therefore, theprocesses of the invention provide an environmentally innocuous accessto valuable REEs, that requires less energy and avoids pollution bytransferring the mining procedure to a controlled containment. Thepresent invention is a break-through in the sustainable exploitation oflow-grade REE-sources, allowing the recovery of REEs in a simpleprocess.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Microbial strains that selectively enrich Scandium. (A)Enrichment factors A (black bars) were calculated byA=[m_(BM)(Sc)*m_(L)(REE)]/[m_(BM)(REE)*m_(L)(Sc)]; m being the mass ofscandium (Sc) or total REE (REE) in biomass (BM) and mineral leach (L),respectively. Recovery W (grey bars) was calculated byW=100*m_(BM)(Sc)/m_(L)(Sc). Assays were carried out as described inExample 2 using a sulphuric acid leach of REE-containing bastnaesite.Designations on the x-axis refer to different microbial isolates.

FIG. 2. Enrichment of specific REE from mineral leach by exemplarymicroorganisms. REE composition of mineral leach as obtained by thetreatment described in example 1 (black bars). Composition of REEextracted by the use of microbial biomass as described in Example 2. They-axis refers to fraction of total REE in percent by weight. A. StrainS3_12G_D6 strongly enriches scandium (A=438) Strain S3_8B_B2 enrichesscandium to a lower extent (A=61), but also significantly accumulatesthe heavy REE lutetium (A=22). B. Composition of REE extracted by use ofCupriavidus metallidurans and Citrobacter sp. from an equimolar solutionof 16 SEE.

DETAILED DESCRIPTION OF THE INVENTION

The REE that may be isolated or enriched in the process of the inventionis selected from lanthanium, cer, praseodym, neodym, promethium,samarium, europium (light rare earth elements, LREE) and scandium,yttrium, gadolinium, terbium, dysprosium, holmium, erbium, thulium,ytterbium, and lutetium (HREE). In one embodiment, the REE is selectedfrom the group of light REEs, i.e. lanthanium, cer, praseodym, neodym,promethium, samarium, europium. In another embodiment, the REE isselected from the group of heavy REEs, i.e. scandium, yttrium,gadolinium, terbium, dysprosium, holmium, erbium thulium, ytterbium,lutetium. In a further embodiment, the REE is selected from the groupconsisting of terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, scandium, yttrium. Scandium is most preferred as the REE.

In the processes of the invention, groups of two or more REEs may beisolated or enriched. Such group may contain two or more REEs from anyof the above-mentioned lists of REEs. A group of REEs may comprise twoor more REEs from the light REEs or from the above list of heavy REEs.

The abbreviation “REE” stands for rare earth element. Multiple rareearth elements are abbreviated by “REEs”. The term “REE(s)” covers themeaning of “REE” and “REEs”. Herein, the term “REE” covers rare earthelements in elemental (metallic) form and chemical compounds comprisingionically or covalently bound REE ions or atoms. Dissolved ions of REEse.g. in aqueous solution are also covered by the term “REEs”. In theprocesses of the invention, the chemical form of the REE may change. Thechemical form of the REE isolated in step (iv) may be different from thechemical form of the REEs in the source material of the REE. Frequently,the chemical form of the REE(s) will also be different in the solutionof step (i) and in the form isolated in step (iv). The difference may bein terms of oxidation state and/or in terms of counter ions to cationicREE ions. It is possible that the solution or suspension used in theprocess of the invention contains REE(s) to be isolated in two or moredifferent chemical states or compounds. Similarly, the REE(s) isolatedin step (iv) may contain said metal in two or more different chemicalstates or compounds.

In natural resources such as in mineral ores, but also in mineral miningwastes, REEs usually occur as REE compounds such as complexes whereinthe REE is present in oxidised form. As is known to the skilled person,the most prevalent oxidation states of REEs are the states +III and +IV.REEs in elemental (metallic) form are easily oxidised to form oxidisedcompounds, and generally react with mineral acids such as dilutesulfuric acid to form hydrogen and REE ions. Due to the stability of theoxidised state of the REEs, the REEs isolated in step (iv) of thepresent invention is generally a REE compound wherein the REE is presentin oxidised form. Thus, the processes of the invention are, in oneembodiment, processes of isolating compounds of REE(s). Known techniquesmay be used for preparing REE in elemental (metallic) form from REEcompounds isolated in step (iv).

In the processes of the invention, the REEs generally bind to thebiomass in the oxidised form of the REEs. Preferably, the REEs arecontained in the mixture prepared in step (i) or (i′) in dissolved form.This does not exclude that REE compounds finely dispersed in the mixtureas small or colloidal particles are also adsorbed or bound by thebiomass. Further, as soluble REE is preferably bound by the biomass instep (ii), insoluble forms of the REE may dissolve according to theirequilibrium solubility under the conditions used.

The solution or dispersion containing the REE or the group of REEs is aliquid. The liquid phase is made up or comprises of a solvent. Thesolvent of the solution or dispersion may be water or an organic solventor mixtures thereof. The organic solvent may be a polar solvent or anon-polar solvent, but is preferably a polar solvent. However, thesolvent is preferably water or contains water, i.e. is aqueous. Theaqueous solvent contains water and may additionally contain a polarorganic solvent. Preferably, the aqueous solvent contains at least 50%by mass water. The solvent is chosen such that a desired degree ofsolubility of the REE compounds to be isolated from the solution ordispersion is obtained. Since the solubility of REE compounds insolvents such as water and other aqueous solutions is generally higherin the acidic range, the solvent may contain an acid. Inorganic(mineral) acids such as sulfuric acid or hydrochloric acid arepreferred, but organic acids may also be used alone or together withinorganic acids. In aqueous solutions or dispersions, the pH may be from0 to 5, preferably from 0.5 to 3, more preferably from 1.0 to 2.0. Uponaddition of the biomass in step (i), the pH or acidity may change.Accordingly, the pH or acid content may be readjusted after addition ofthe biomass in order to maintain the desired acidity or pH for step (i)of the process. The mixture of step (ii) may have a pH of from 0 to 5,preferably of from 0.5 to 3, more preferably of from 1.0 to 2.0.

The solution or dispersion containing said REE or group of REEs to beused in step (i) may be obtained or prepared in different ways thatdepend to the source material of the REE(s). In one embodiment, theREE(s) are already present in the form of a solution or dispersion inthe source material, such as in liquid tailings from mining industry orliquids that form from rain falling on piles of solid mining wastematerial and washing out REE(s) from the waste material. In such cases,the solutions or dispersions may be used for step (i) as they are,optionally after adjustment of conditions such as concentration and/oracidity.

In another embodiment, the source material of the REE(s) is metal orelectronic scrap that may contain the REE(s) in metallic elemental formor chemically bound or both. Metallic REE(s) may be separated andenriched mechanically. Metallic REE(s) to be isolated using the processof the invention are generally transformed chemically to soluble form,e.g. by treatment with aqueous organic or mineral acids, such assulfuric acid or hydrochloric acid to form the respective REE salts ofsuch acids, such as sulfates or chlorides, respectively, in aqueoussolution or dispersion. Also REE compounds that are insoluble or poorlysoluble in water at neutral pH may be solubilised by such acids. Thesolutions or dispersions of the REEs used in step (i) may be acidic asdescribed above, since the solubility of REE salts is generally higherin acidic solutions. The pH and other conditions may be adjusted asrequired for step (ii).

In other embodiments, the source material of the REE(s) are mineral oresor solid mine waste material obtained from processes of producing otherdesired components from ore. In these cases, the source material isgenerally first ground to fine particulate material for improvingaccessibility and leachability of REE(s) contained therein. Examples ofmineral ores containing REE(s) are bastnaesite, thortveitite, monazite,loparite, gadolinite, euxerite, eschynite, allanite, apatite,britholite, brockite, cerite, fluorcerite, fluorite, parisite,stillwellite, synchisite, titanite, xenotime, zircon, zirconolite, etc.The mineral ores generally contain the REE(s) in the form of chemicalcompounds such as carbonates, sulfates, oxides, phosphates or silicates.To ensure efficient solubilisation and extraction of the REEs from theores or mining waste material, these should be finely ground to formparticulate material. The particulate material may have an averageparticle size of at most 5 mm. Alternatively, the particulate materialmay have an average particle size of at most 1 mm, of at most 400 μm, orof at moat 100 μm, or of at most 50 μm, or of at most 30 μm.

For preparing the solution or dispersion for step (i) from mineral oresor mining waste material as source material, the preferably comminutedparticulate material may be extracted (leached) with suitable solventssuch as with organic solvents, bases or organic or inorganic acids,preferably with inorganic acids such as with dilute sulfuric acid orhydrochloric acid, whereby the REE compounds are dissolved in thesolvent. The acid concentration in the dilute organic or mineral acid isnot particularly limited, but should be at least 2% by weight forensuring sufficient efficiency. The concentration may be from 5 to 20%by weight or from 7 to 15% by weight. Alternatively, other methodssuitable to facilitate REE release and solubilisation may be employed,such as oxidative or heterotrophic bioleaching or incubation withmicroorganisms that produce corrosive metabolites. After the extraction,solid material may be removed e.g. by filtration or sedimentation.Furthermore, bioleaching procedures may be used to release the REE(s) ofthe invention from the particulate material. Autotrophic orheterotrophic bacteria may be used for pre-treating the source materialor the particulate material for bioleaching. For instance, if saidparticulate material is a sulfidic ore such as pyrite, sulfide-oxidizingbacteria such as Thiobacilli may be used for at least partiallydegrading the sulfidic mineral. Such bioleaching is described byRawlings, D E and Johnson, D B (The microbiology of biomining:development and optimization of mineral-oxidizing microbial consortia.Microbiology 2007, 153: 315-24). Bioleaching may be carried out beforeor after solvent extraction. If it is carried out after solventextraction such us with strong acid or bases, a step of bringing theacid or base content of the particulate material to a level suitable forthe bacteria used for bioleaching may be necessary. In step (i′),bioleaching may be done concurrently with step (i′) and subsequent step(ii) by adding the bacteria for bioleaching to the biomass of step (i′).In one embodiment, the sulfide-oxidising bacteria aregenetically-modified to express an S-layer on the surface of saidbacteria.

In step (i) of the process of the invention, a mixture is prepared fromsaid solution or dispersion containing the REE or group of REEs to beisolated and said biomass in step (i′), a mixture is prepared from thebiomass and a solution or dispersion obtained from the particulatematter, whereby the particulate matter may still be present in themixture. If the particulate matter is still present, extraction and/orbioleaching may take place or continue to take place concomitantly withbinding of the REE(s) by the biomass in step (i′) and subsequent step(ii). If the acidity of the solution from a previous leaching step istoo high, the pH may be increased by addition of bases to reach a pHthat is compatible with the biomass used in steps (i), (i′) and (ii).Preferred pH ranges for these steps are given below.

Steps (i) and (i′) and subsequent step (ii) may be conducted in closedreactors that preferably contain an agitation system for agitating themixture. The reactor may be a stirred-tank reactor and may be operatedin a batch or continuous-flow mode. The reactor is preferably equippedwith devices for measuring and controlling process parameters such astemperature, pH, etc.

The biomass used in step (i) or (i′) comprises organisms, preferablymicroorganisms, selected from Eubacteria, Archaea, Algae, and Fungi.Among these, Eubacteria, Archaea and Algae are preferred. Eubacteria andArchaea are more preferred and Eubacteria are most preferred. Themicroorganisms used may naturally or by state-of-the-art geneticengineering have the potential to bind REEs. Generally, the REEs arebound in the oxidised term of the REEs. This property is exploited toadsorb or accumulate REEs that are present in the mixtures of stops (i),(i′) and (ii). The organism to be used depends on the type of REE orgroup of REEs to be isolated. Other criteria for the choice of thebiomass may be the chemical state of the REE present in the mixtures.Suitable organisms for a given REE or REE compound can be identified byscreening large strain collections using the procedure described belowand in Example 1. Screening may be done following the procedure ofExample 1 or Example 2. In the research that led to the invention,microorganisms were assayed for their ability to grow in the presence ofREEs and, in a secondary screening, to bind and/or accumulate REEs. Analternative to screening a broad diversity of organisms is thepre-selection of microbes that belong to phylogenetic groups that haveturned out to have high metal-binding potential. The biomass may bindthe REEs by adsorption to the cell surface or cell wall opponents, orvia active intracellular accumulation of ions of the REEs.Microorganisms carrying homologous or heterologous metal-binding ormodifying structures such as S-layers, polysaccharides, metal-reducingenzymes, metallothioneines, phytochelatins or surface-bound naturalmetallophores are suitable organisms for the present invention.

Microorganisms, particularly eubacteria and archaea, fungi and algaethat have the required REE-binding affinity and specificity can beisolated from environmental sources, using known microbiologicaltechniques. Environmental sources (habitats) that contain organismssuitable for the present invention are, however not exclusively,sediments and waters exposed to heavy metal or radionuclidecontamination, such as acid mine drainages, electroplating effluents,mining waste piles, industrial effluents, and waste water treatmentplants. Microorganisms viable and competitive in these environmentsoften have adopted strategies to efficiently bind and immobilize heavymetals either on their surface or in their interior in order to reducetheir toxicity. Typically, cell envelopes of microorganisms exhibitnegative charges, enabling the adsorption of cationic metals. The mainfunctional groups that contribute to this negative charge are phosphatemoieties and carboxylic groups.

Organisms that do not naturally have metal-binding components can beprovided with components allowing binding of the REE of the invention bygenetic engineering. For instance, DNA fragments encoding genes orpathways that lead to the formation of metal-binding ormetal-immobilizing structures can be introduced into wild-type strains,using techniques known to those skilled in the art. The presentinvention makes use of microorganisms that naturally—or by geneticengineering—have the potential to bind REEs.

Examples of natural or genetically-engineered components of organismsthat may be used for binding the metal of the invention are thefollowing.

Metallothionines. These metal-chelating polypeptides have beenidentified in many groups of organisms, including mammals, nematodes,fungi, and bacteria. Metallothioneines are characterized by an extremelyhigh cysteine content of up to 33% arranged in (Cys-X-X-Cys) or(Cys-X-Cys) clusters and the absence of aromatic and hydrophobic aminoacids.

Phytochelatins. Phytochelatins typically occur in plants and algae andare short, non-translationally synthesized polypeptides with variouslyrepeating gamma-glutamylcysteine units (γGlu-Cys)_(n)Gly (n=2-11).Synthetic phytochelatins [(glu-Cys)nGly] nave the advantage that theycan be synthesized by the ribosomal machinery and that in some casesthey bind metals even more effectively than the natural phytochelatins.

S-layers. Paracrystalline proteinaceous surface layers (S-layers) occuras surface structures in almost all major phylogenetic groups ofbacteria and in almost all archaea (Sara and Sleytr 2000). The proteins(40-200 kDa) are secreted and subsequently self-assemble on thebacterial membrane, forming a very regular nano-porous structure (30-70%porosity). S-layer proteins constitute up to 20% of all cellularproteins. Due to their high content in hydrophobic amino acids, S-layerlattices in general render prokaryotic cell walls less hydrophillic,which can lead to increased foaming during cultivation. Immobilizationof metals on S-layer templates has been used in nanotechnology tosynthesize metallic nanoclusters of the precious metals Au (Dieluweit,Pum et al. 1998; Györvary, Schroedter et al. 2004) and Pt and Pd (Wahl,Mertig et al. 2001).

Polysaccharides. Some microorganisms produce biopolymers, e.g.polysaccharides that are able to bind 0.1 mg to 1.4 g metal/g isolatedpolymer, depending on the microorganism under investigation and thespecific metal (Gutnick and Bach 2000). Binding generally occurs viaelectrostatic interactions between negatively charged groups in thebiopolymer and the positively charged metal or via chelation of themetal by hydroxyl groups.

Examples of suitable microorganisms to be used in the present inventionare microorganisms from genus Cupriavidus, such as Cupriavidusmetallidurans. An example of Cupriavidus metallidurans is DSMZ Typestrain 2839. Another example is Citrobacter sp. These microorganisms arepreferably used in the methods and uses of the invention for isolatingor enriching scandium.

In the process of the invention, it is possible to combine two or moremicroorganisms in the biomass. For example, different microorganismseach preferentially binding a particular REE (or groups thereof) may becombined for increasing the variety of REEs that may be isolated in theprocess. Depending on the composition of the particulate material, twoor more microorganisms can be combined in said biomass to recoverdifferent chemical forms of REEs or different REEs in parallel.

In one embodiment, microorganisms for the processes of the invention areeubacteria and archaea. Microorganisms of the genera of Pseudomonas,Cupriavidus or Bacillus are preferred. A preferred species from genusCupriavidus is Cupriavidus metallidurans such as DSMZ type strain 2839.

In another embodiment, the biomass used in the invention is or containsan organism belonging to eubacteria or archaea for adsorbing oraccumulating scandium as the REE. For this purpose, microorganisms ofthe genera of Pseudomonas, Cupriavidus or Bacillus may be used. Apreferred species from genus Cupriavidus is Cupriavidus metalliduranssuch as DSMZ type strain 2839 for scandium isolation.

The biomass used in the invention may be viable or dead. Native cells asobtained by cultivation in growth media (wet biomass) as well as drybiomass, e.g. obtained by freeze-drying or by drying at elevatedtemperatures can be used. Temperatures applied during drying should notexceed 100° C. in order to prevent thermal degradation of cellcomponents that are involved in specific REE adsorption. Preferably,however, the biomass used in step (i) and (i′) is viable, i.e. containsviable cells of the organisms used. The conditions in the mixtures ofsteps (i), (i′) and (ii) may be such that the organisms in the mixtureremain viable to a large extent and may even grow further in the step(ii). In an embodiment where the organisms of the biomass should stayviable in step (ii), conditions have to support viability. For thispurpose, the mixtures may contain nutrients required for the biomass.Further, air by be blown into the mixtures for providing oxygen to thebiomass. Suitable growth conditions and nutrient requirements for theorganisms can be obtained from the general prior art on microbiology.Suitable growth conditions are also provided by collections ofmicroorganisms such as the American Type Culture Collection (ATCC) orthe German Collection of Microorganisms and Cell Cultures (DeutscheSammlung von Mikroorganismen und Zellkulturen GmbH, DSMZ) where membersof the classes of microorganisms mentioned above can be obtained from.

In another embedment, the biomass added in step (i) or (i′) is viable,but is allowed to fully or partly die in the course of the process.Generally, the biomass may at least partly die due to unfavourableconditions in the mixture such as acidic pH.

In step (ii) of the processes of the invention, the mixture of step (i)or (i′) is incubated for allowing binding such as adsorption oraccumulation, of said REE or said group of REEs by said biomass. Step(ii) should be conducted for a time period sufficient to allow thebiomass to adsorb and/or accumulate the REE(s) from the solution. Theincubation time may be chosen such that no or little more REE(s) areabsorbed or accumulated by the biomass at the end of the incubationstep. The incubation time depends on the rate of binding. Generally, theincubation time is between 0.5 hours and 96 hours, preferably between0.5 hours and 48 hours, and more preferably between 1 hour and 24 hours.The temperature of incubation depends mostly on the type of biomass usedand the REE that are to be recovered. Step (ii) may comprise agitatingsaid mixture for bringing said biomass in close contact with particlesof said particulate material. As indicated above with respect to step(i), the reactor in which step (ii) is carried out may be equipped withdevices for measuring and controlling process parameters such astemperature, pH, etc.

In step (iii), said biomass having bound REE(s) is separated from themixture of step (ii). Known methods may be used for the separation. Forexample, the metal-loaded biomass may be separated from the solution bycentrifugation or filtration. Alternatively, flocculation or flotationmay be used. The separated biomass may, depending on the subsequentstep, be dried for facilitating storage and/or transport and/or metalseparation in step (iv).

In step (iv), the metal bound to the biomass is isolated from saidbiomass. The metal may for example be desorbed from the biomass in aliquid phase using acidic or basic conditions, or elution with chemicalssuch as chelating agents that can form soluble complexes with the REEs.Alternatively, the biomass may be combusted to destroy and removeorganic matter of said biomass, whereby the REE can be isolated fromashes or fumes. Further, mechanical means may be used for separating theREEs from the biomass, such as sonication. The REE(s) may be purifiedfrom the residues, ashes or fumes of the biomass. Preferably, theisolation method allows recycling of the biomass for use in furtherREE-extraction processes.

In one embodiment, the mixture from step (ii) may be poured in step(iii) into chromatography columns that holds back the biomass but allowsremoval of excess liquid from the column. In this embodiment, step (iv)may be performed by eluting REEs from the column using a liquid mediumas eluent that weakens the binding of the REEs to the biomass such ascomplexing agents. In this way, the REEs, notably soluble compoundsthereof, may be obtained in concentrated eluent.

The processes of the invention may be combined with process steps usedfor isolating specific REE(s) from metal solutions known from prior art.For example, where a group of REEs was isolated in step (iv), individualREEs or compounds thereof may be isolated. If desired, REEs in elementalform may be generated from REE compounds by known reduction methods.

The invention also provides a method of testing a sample microorganismfor its ability to bind a REE, and a method of screening a plurality ofmicroorganisms for the ability of members of said plurality to bind arare earth element comprising the following steps:

-   -   (a) contacting a microorganism with a solution or dispersion        containing a rare earth element or multiple REEs;    -   (b) incubating the mixture of step (a) for a predetermined        period of time;    -   (c) separating microorganisms from the mixture obtained in step        (b);    -   (d) analysing the separated microorganism for bound REE.

In step (a), a microorganism is contacted with a solution or dispersioncontaining a REE or multiple REEs. Similarly as described above, themicroorganism may be selected from the following organism classes:Eubacteria, Archaea, Algae, and Fungi. The REE may be solution,preferably an aqueous solution, of the REE for which the binding abilityof the microorganism is to be tested. Preferably, the solution containstwo or more REEs, since this allows testing the binding ability of themicroorganism to multiple REEs in parallel if the separatedmicroorganism is analysed in step (d) for the multiple REEs.

For increased solubility of the REE in the solution, the solution maycontain an acid such as those mentioned above. The solution ordispersion used in step (a) may be an acidic aqueous solution ordispersion. The pH of the solution or dispersion may be from 0 to 5,preferably of from 0.5 to 3, more preferably of from 1.0 to 2.0. The pHof the mixture of step (ii) may be from 0 to 5, preferably from 0.5 to3, more preferably from 1.0 to 2.0.

In incubation step (b) and the separation step (c) may be carried out asdescribed above for steps (ii) and (iii) of the process of isolating aREE.

In step (d), the separated microorganism is analysed for bound REE. Thismay involve extraction of metal compounds from the separatedmicroorganism e.g. using a mixture of concentrated nitric acid andhydrochloric acid, preferably in combination with heating. Theextraction acid may then be diluted with pure water and subjected to anygenerally known method for analysing REEs such as atom absorptionspectroscopy or inductively coupled plasma mass spectroscopy (ICP-MS).These methods may be used for quantitative analysis of REEs in theseparated microorganism. If multiple REEs are detected, their relativeabundance may be compared with the relative abundance of multiple REEsin the starting REE solution. Thus, enrichment of one or more particularREEs compared to others can also be detected. In this way, a suitable oroptimal microorganism for a particular purpose may be found.

EXAMPLES Example 1 Preparation of Mineral Leach

Bastnaesit ore was leached using 10%-H₂SO₄ in a 15 (w/v) ratio of oreand acid. A 50-g sample of ore was incubated under continuous stirringwith 250 ml of 10%-H₂SO₄ for 3 days at room temperature. The leachsolution was centrifuged to remove non-dissolved particles and thesupernatant was used as so-called “mineral leach” for furtherexperiments. Before each experiment, the pH of mineral leach wasadjusted to 1.3 by the addition of 10N NaOH.

Example 2 Screening for Microorganisms that enrich Scandium from MineralLeach

Microbial strains that have the potential for adsorbing/accumulating REEwere selected by their ability to grow on solid media (Luria Bertani(LB) medium 10 g/l tryptone and 5 g/l yeast extract, with 1.5 g/l agar)containing amounts of 1 to 5 mM REE (single elements or mixtures).REE-resistant microorganisms were screened for their ability to enrichscandium (Sc) from mineral leach. To this end, microbial strains werecultivated in LB medium (without agar) at the 50-ml scale according tostandard microbiological techniques. Cells were collected in thestationary phase by centrifugation. Amounts of 20 OD units wereincubated for 1 h at room temperature (25° C.) with 1 ml of mineralleach. After incubation, cells were collected by centrifugation andwashed once with 100 μl 10%-H₂SO₄. As a control for spontaneous(chemical) precipitation of REE, 1-ml aliquots of mineral leach withoutbiomass were used. Cell pellets and precipitates were extracted bynitrohydrochloric acid for 2 h at 100° C. using a Digi-Prep samplepreparation device (S-Prep, Oberlingen, Germany). After dilution inultrapure water, REE contents of the cell pellets were determined byICP-MS (Agilent, 7700 ICP-MS).

Results

A number of 36 microbial strains (hit candidates) were detected thatselectively bind Sc in their biomass. Enrichment factors for Sc comparedto the other REE of up to 438 were observed (FIG. 1). Analysis ofpartial 16 S rDNA sequences suggested that many hit candidates originatefrom the groups of Pseudomonas and Bacillus.

Example 3 Scandium Recovery from Mineral Leach by Microbial Biomass

Strains S3_8B_D12 and S3_8B_B2 were used to determine Sc recovery in anexperimental set-up as described in Example 2. As shown in FIG. 2 A,S3_8B_D12 was able to enrich Sc by a factor of 438 compared to theoriginal mineral leach (black bars, REE=w/w), leading to a REE mixturethat contains more than 80% of the target element. By the appliedsingle-step extraction, 33% of the present Sc could be recovered. Withstrain S3_8B_B2 only an enrichment factor of 61 could be achieved for Sc(W=22%). On the other hand, however, also Lutetium—a veryunderrepresented heavy REE—could be enriched by a factor 22, leading toa recovery of 8% of the present material.

We used DSMZ Type strain 2839 (Cupriavidus metallidurans) andCitrobacter sp. to recover REE from an equimolar solution (1 mM each, in10%-H₂SO₄; pH adjusted to 2.2 by addition of 1N-NaOH) of all 16 stableREE (FIG. 2B, black bars=SEE-mix, percent (w/w)). 20 OD units of cellsoriginating from a stationary phase (overnight) culture prepared in LBmedium were incubated for 1 h with 1 ml of said REE solution. Afterincubation, cells were collected by centrifugation and washed once with100 μl 10%-H₂SO₄. ICP-MS analysis was carried out as described above.

The content of European patent application No. 13 003 058.8, filed onJun. 14, 2013 is herewith incorporated by reference in its entiretyincluding entire description, claims and figures.

REFERENCES

-   Dieluweit S., D. Pum, et al. (1998). “Formation of a gold    superlattice on an S-layer with square lattice symmetry.” Supramol    Sci 5: 15-19.-   Györvary. E., A. Schroedter, et al. (2004). “Formation of    nanoparticle arrays on S-layer protein lattices.” J Nanosci    Nanotechnol 4(1-2): 115-20.-   Gutnick, D. L. and H. Bach (2000). “Engineering bacterial    biopolymers (or the biosorption of heavy metals: new products and    novel formulations.” Appl Microbiol Biotechnol 54(4): 451-60.-   Sara, M. and U. B. Sleytr (2000). “S-Layer proteins.” J Bacteriol    182(4): 859-68.-   Vieira, R. H. and B. Volesky (2000). “Biosorption: a solution to    pollution?” Int Microbiol 3(1): 17-24.-   Wahl, R., M. Mertig, et al. (2001). “Electron-beam induced formation    of highly ordered palladium and platinum nanoparticle arrays on the    S-layer of Bacillus sphaericus NCTC 9602.” Adv Mater 13: 736-740.

1. A process of isolating or enriching the rare earth clement (REE)scandium from a solution or dispersion containing said REE, comprisingthe following steps: (i) preparing a mixture comprising said solution ordispersion and biomass comprising at least one organism selected fromany one of the following organism classes: eubacteria, archaea, algae,and fungi, whereby the at least one organism is capable of adsorbing oraccumulating said REE; (ii) incubating said mixture of step (i) forallowing the adsorption or accumulation of said REE by said biomass;(iii) separating the biomass having adsorbed or accumulated REE(s) fromthe mixture of step (ii); and (iv) isolating said REE from said biomassseparated in step (iii).
 2. The process according to claim 1, whereinsaid solution or dispersion is obtained by treating a source material ofsaid REE, such as a mineral ore, a mineral mining waste material, or anelectronic or metal scrap, by an extraction or solubilizing agent, suchas an aqueous acid.
 3. A process of isolating or enriching the rareearth element (REE) scandium from a particulate material, comprising thefollowing steps: (i) preparing a mixture comprising a solution ordispersion prepared from said particulate material and biomasscomprising at least one organism selected from any one of the followingorganism classes: eubacteria, archaea, algae, and fungi, whereby the atleast one organism is capable of adsorbing or accumulating said REE;(ii) incubating said mixture of step (i) for allowing adsorption oraccumulation of said REE by said biomass; (iii) separating the biomasshaving adsorbed or accumulated REE(s) from the mixture of step (ii); and(iv) isolating said REE from said biomass separated in step (iii). 4.The process according to claim 3, wherein said particulate materialcontaining said REE is a particulate mineral ore, a particulate mineralmining waste material or a particulate obtained from electronic scrap orscrap metal.
 5. The process according to claim 3, wherein saidparticulate material contains said REE in the form of chemical compoundsof the REE, such as carbonates, sulfates, oxides, phosphates orsilicates.
 6. The process according to claim 3, wherein said particulatematerial has an average particle size of at most 5 mm, preferably atmost 1 mm, more preferably of at most 400 μm, and moat preferably of atmost 100 μm.
 7. The process according to claim 3, wherein said solutionor dispersion is obtained by treating said particulate material by anextraction or solubilising agent such as an aqueous acid, optionallyfollowed by adjusting the pH to a suitable pH for adsorption oraccumulation of said REE by said biomass.
 8. The process according toclaim 3, wherein preparing said solution or dispersion from saidparticulate material comprises pre-treating said particulate materialwith autotrophic or heterotrophic bacteria tor bioleaching said REE fromsaid particulate material before or concurrently to step (i).
 9. Theprocess according to claim 1, wherein said solution or dispersion isaqueous.
 10. The process according to claim 9, wherein said aqueoussolution or dispersion has a pH of from 0 to 5, preferably of from 0.5to 3, more preferably of from 1.0 to 2.0; and/or said mixture of step(ii) has a pH of from 0 to 5, preferably of from 0.5 to 3, morepreferably of from 1.0 to 2.0. 11.-12. (canceled)
 13. The processaccording to claim 1, wherein said organism is from genus Cupriavidus,such as Cupriavidus metallidurans, or said organism is Citrobacter sp.14. A process of isolating the REE scandium from a particulate material,or a solution or dispersion containing said REE comprising

of contacting biomass selected from the following organism classes:Eubacteria, Archaea, Algae, and Fungi with said particulate material ora solution or dispersion containing said REE.
 15. A method of testing asample microorganism for its ability to bind the rare earth elementscandium or of screening a plurality of microorganisms for the abilityof members of said plurality to bind the rare earth element scandium,comprising the following steps: (a) contacting a microorganism with asolution or dispersion containing the rare earth element (REE); (b)incubating the mixture of step (a) for a predetermined period of time;(c) separating microorganisms from the mixture obtained in step (b); (d)analysing the separated microorganism for bound REE.
 16. The processaccording to claim 3, wherein said solution or dispersion is aqueous.17. The process according to claim 14, wherein said aqueous solution ordispersion has a pH of from 0 to 5, preferably of from 0.5 to 3, morepreferably of from 1.0 to 2.0; and/or said mixture of step (ii) has a pHof from 0 to 5, preferably of from 0.5 to 3, more preferably of from 1.0to 2.0.
 18. The process according to claim 3, wherein said organism isfrom genus Cupriavidus, such as Cupriavidus metallidurans, or saidorganism is Citrobacter sp.